China disscusses possible stationing of Chinese troops in N. Korea

they must send troops based on national security concerns . the NK is not stable right now due to the changing leadership , SK might think it is a good chance to reunify the country when Kim Jong Il abdicates , I think that must be what is the US thinking about .

I don't think the DPRK is about to "collapse", as it has a big protector, and maybe two, right on its northern border. In any case, a state does not collapse in an instant, and China would easily know about developments with enough time to react. Moreover, a small contingent guarding some port would basically be powerless to do anything. So there's no advantage to basing troops inside the DPRK, if the issue is the political situation in the country. If this contingent exists, I would expect it is simply to guard installations, or possibly to man electronic intelligence gathering, since so many military drills take place along Korea's eastern coast.